Why are only the core database files backed up by default?

Because it's a fairly safe bet that the core WordPress files will be successfully backed up. Plugins vary wildly in the amount of data that they store. For instance, it's not uncommon for some statistics plugins to have tens of megabytes worth of visitor statistics. These are not exactly essential items to restore after a catastrophic failure. Most poeple can reasonably live without this data in their backups.